The present invention relates generally to the field of analog signal conditioning, and in particular to AC-to-DC converters for converting AC displacement sensor signals to a DC signal.
A linear variable differential transformer (LVDT) provides an electrical output signal that is proportional to the displacement of a separate moveable iron core. An LVDT uses three windings and the moveable iron core to sense linear displacement. A primary winding, two secondary windings, and the moveable iron core are energized at the primary with an alternating current (AC). The two secondary windings are connected in series opposition, such that the transformer output is the difference of the two secondary voltages. When the core is centered, the two secondary voltages are equal and the transformer output is zero. This is the balanced or null position. When the core is displaced from the null point, the two secondary voltages are no longer equal in magnitude and the transformer produces an output voltage. Motion of the core in the opposite direction produces a similar effect with 180xc2x0 phase reversal of the alternating output voltage, i.e., the phase angle is positive (no phase shift with respect to the excitation) or negative (180xc2x0 phase shift with respect to the excitation) depending on which side of null the core is positioned. A demodulator circuit can be used to produce a DC output from this winding configuration. Differential transformers are also available in a rotary version (e.g., an RVDT) for angular measurement in which the core rotates about a fixed axis. A detector is normally used for sensing phase reversal when passing through the null point.
Other winding configurations are used in synchros, resolvers and microsyns.
The construction of a synchro is similar to that of a three-phase synchronous motor or generator. The stator contains a three-phase winding and the rotor is excited with a constant single phase AC voltage while the shaft moves at low speeds or stays stationary. Basically the synchro is a transformer with one primary (the rotor) and three-secondaries (the Y-connected windings of the stator). The voltages induced in the secondary windings are proportional to the cosines of the angles between each stator coil and the rotor.
A resolver synchro is similar to a synchro generator in construction, but the stator contains only two windings oriented at 90xc2x0 relative to each other, and they are employed to resolve rotor position into sine and cosine component voltage signals.
The various types of displacement sensors may be used in computing servomechanisms and other electromechanical computers. When used in digital computers, it is often necessary to convert the analog signal information into digital words for use by the signal processor. Usually, a rectification process is utilized to convert time-varying secondary output signals, having a zero average, into a rectified signal having a DC average value. Unfortunately, this process destroys the phase information contained in the secondary output signals. Unless the information is extracted before rectification and later converted to useful digital information as well, the displacement sensor will necessarily be restricted to operation in a limited range. Thus, an LVDT would be restricted to use on one or the other side of the null point while a synchro resolver would be restricted to operation in one quadrant only.
U.S. Pat. 4,561,130 assigned to the assignee of the present invention discloses an apparatus and method for retaining both amplitude and phase information in the signal derived from the secondary coils of a multiple-coil inductive displacement sensor and applied to a rectifier for conversion to a digital format. This technique involves summing the time varying input with a reference time varying signal. The resultant summed signal is then rectified and digitized. The reference signal is also rectified and digitized. The processor reads the digital value of the converted reference signal and the digital value of the converted summed signal and computes the difference to provide a signed digital value. Significantly, although this technique provides both signal phase and amplitude with the signed digital value, this technique requires an analog-to-digital converter (ADC) and a processor to perform the subtraction.
Therefore, there is a need for a simpler and less expensive technique for recovering the transducer output signal phase and amplitude information.
Briefly, according to an aspect of the present invention, an AC-to-DC converter obtains phase and amplitude signal information from a displacement transducer that is excited by an AC excitation signal and provides an AC sensor signal indicative of transducer position. The converter includes a first rectifier circuit that receives and sums the AC excitation signal and the AC sensor signal, and rectifies the sum to provide a rectified summed excitation and input signal indicative thereof. A second rectifier circuit receives and rectifies the AC excitation signal, and provides a rectified excitation signal indicative thereof. A summing circuit computes the difference between the rectified summed excitation and input signal and the rectified excitation signal, and provides a signed DC signal indicative of displacement transducer position.
Advantageously, the AC-to-DC converter of the present invention performs summing and difference functions directly and provides a signed DC signal representative of the AC amplitude and phase input of the transducer output, thus eliminating the need for an analog-to-digital converter (ADC) and the support of an associated processor.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.